This invention relates to routing optical signals using optical waveguides located in layers of interconnect optical waveguides in an integrated circuit (IC)-like structure. More particularly, this invention relates to a new and improved optical waveguide having the capability to route an optical signal in horizontally deflected directions within the same waveguide. The invention also relates to a new and improved method of fabricating the optical waveguide in an IC-like structure using damascene fabrication process steps that are typically employed in the fabrication of electrical integrated circuits.
This invention is related to the inventions for a xe2x80x9cOn-Chip Graded Index of Refraction Optical Waveguide and Damascene Method of Fabricating The Samexe2x80x9dand xe2x80x9cOn-Chip Multiple Layer Transitioning Optical Waveguide and Damascene ofMethod of Fabricating the Same,xe2x80x9d described in concurrently filed U.S. patent application Ser. Nos. 09/217,183 and 09/217,184, respectively. These applications are assigned to the assignee hereof. The subject matter of these applications is incorporated herein by this reference.
The ongoing evolution of microcircuit design has focused on the speed and size of electrical integrated circuit (IC) components, typically in a silicon chip. IC designers have continuously strived to make the IC faster while taking up less chip space. Currently, interconnection technology is considered as one of several areas that may be advanced to both increase the speed of the IC and to decrease the size of the chip. For instance, since most of the conductors that interconnect various functional components on the chip are made of metal and carry electrical signals, advances are being made in various metal compositions that can carry similar signals at a faster speed but which are smaller and thus consume less space.
Optical signals carried by waveguides are sometimes considered as replacements to the more common electrical signals carried by metal conductors. Optical signals allow the IC to operate more quickly or at a higher speed, and unlike electrical signals, optical signals are usually not susceptible to noise and interference. In general, optical conduction and reduced susceptibility to noise and interference obtain increased speed in data transmission and processing.
Furthermore, due to the coherent nature of laser optical signals and their reduced susceptibility to noise, many more optical signals can be routed in one waveguide or layer of waveguides than is possible using conventional electrical signal interconnect conductors. Therefore, an IC-like structure incorporating optical interconnect waveguides may have fewer waveguides and consume less space.
One typical type of optical interconnection between two IC components comprises a single waveguide or channel between the two components. In general this single waveguide is a straight conductive path between conversion devices which convert electrical signals to optical signals and convert optical signals to electrical signals.
Another type of controllable optical interconnect is called a xe2x80x9crailtap.xe2x80x9d A railtap comprises a first conversion device that converts an electrical signal from a first IC component to an optical signal, an interconnect waveguide that conducts the optical signal from the first conversion device to a second conversion device, where the second conversion device converts the optical signal to an electrical signal and applies it to the second IC component. Upon receiving an electrical signal from the first component, the railtap diverts an optical light signal from a light source waveguide into the interconnect waveguide. An active waveguide polymer is connected to electrodes, and the electrodes create an electric field about the active waveguide polymer, causing a change in the index of refraction of the polymer, usually making it closer to the index of refraction of the source waveguide. When the index of refractions of the railtap and the source waveguide are similar, light is refracted from the source into the railtap polymer. Light is thereby transmitted selectively through the interconnect waveguide toward the second component as a result of applying the electric field to the electrodes on the active waveguide polymer.
The typical waveguide is formed of light transmissive material which is surrounded by an opaque cladding material. Optical signals propagate through the channel, guided by the cladding material. As the optical signals propagate through a particular waveguide, the signals impinge on the cladding material. If the index of refraction of the cladding material is lower than the index of refraction of the material within is channel, the majority of the impinging light signal reflects from the cladding material and back towards the center of the channel. Thus the signal propagates through the channel as a result of reflection at the interface of the cladding material.
On the other hand, if the index of refraction of the cladding material is equal to or greater than the channel material, the impinging light signal tends to refract into the cladding material, thus drawing some or all of the optical power of the light signal out of the waveguide. As more light is drawn out of the waveguide, the intensity of the signal received from the waveguide is reduced. An ideal, lossless waveguide propagates an optical signal without losing any signal intensity through refraction.
The physical placement of the various functional components in the substrate of the IC-like structure and in its interconnect layers generally requires flexibility in layout. Optical waveguides used as interconnects in IC-like structures are formed as singular straight channels, since light signals do not bend around corners. The channel can bothnot incorporate any bends or corners because the light signal will not follow the channel. Once an optical signal propagates the length of the straight channel, a directional coupler must redirect the optical signal if a change in direction is desired. Direction couplers substantially increase the manufacturing cost and size of the IC-like structure. The requirement that the optical waveguides extend in straight lines is a substantial disadvantage in the layout of optical IC-like structures, unlike electrical ICs where the electrical conductors can be routed in essentially any direction and shape to accommodate various aspects of circuit layout.
It is with respect to these and other considerations that the present invention has evolved.
An aspect of the present invention relates to guiding an optical signal in a changed direction using only an optical waveguide within a single optical interconnect layer of an IC-like structure. Another aspect relates to splitting an optical signal into two separate signals and propagating those signals in different directions within a single optical interconnect layer of an IC-like structure. A further aspect of the present invention relates to improving the ability to lay out optical waveguides as optical interconnects in an IC-like structure. A further aspect relates to fabricating optical waveguides having the above-noted capabilities using known damascene process steps already used to fabricate electrical ICs.
In accordance with these and other aspects, the invention relates to-a horizontal deflecting optical waveguide formed in an integrated circuit-like structure having a substrate. The optical waveguide includes at least one layer of dielectric material positioned above the substrate, and a trench in the dielectric material which has side walls. The trench has first and second portions joined at a bent portion where the first and second portions extend in an angular relation with respect to each other. A reflective layer of material adjoins, conforms to, and extends along the side walls of the first, second and bent portions of the trench. A core of optically transmissive material adjoins and conforms to the reflective layer within the trench. The reflective layer forms a wall at the bent portion which reflects light from the core located in one portion into the core located in the other portion.
In accordance with other aspects, the invention also relates to a method of fabricating a horizontal deflecting optical waveguide in interlayer dielectric material located above the substrate of an integrated circuit-like structure. The method involves forming a trench having first and second portions joined at a bent portion in the dielectric layer, where the first and second portions extend in an angular relation with respect to one another, forming a wall in the trench which extends at an angle with respect to the first and second portions, forming a reflective layer of material in the trench and on the wall,. and forming a core of optically transmissive material within an interior of the reflective layer.
Other preferred aspects of the invention relate to a beam splitter. At least three segments of the waveguide are formed and connected. A wall extends into the trench at the junction of the three segments, and a reflective layer of material is formed on the wall. The reflective layer on the wall reflects light from the core of one segment into the cores of the other two segments.
Other preferred aspects of the invention relate to the following features described in this paragraph. The core is surrounded by the reflective layer except on one side, and the index of refraction of the core is greater than the index of refraction of the dielectric material on the side where the core is not surrounded by the reflective layer. The reflective layer may be formed in a U-shaped configuration to facilitate depositing the reflective layer in the trench in a self-aligning manner. A cap of reflective material may be deposited and formed between ends of the U-shaped reflective layer, causing the cap and the U-shaped reflective layer to encircle the core. The reflective layer is integrally continuous over the side walls of the trench and the wall at the bent portion which causes the reflection. The core material is also deposited in a self aligning manner with regard to the reflective layer.
As a result of these new and improved features, optical waveguides may be employed more effectively in interconnect layers of an IC-like structure, because they may be bent and oriented in configurations other than straight lines, without using separate functional directional couplers. Furthermore, optical signals may be split or combined without using separate functional devices to accomplish the splitting or combination. The steps to fabricate the optical waveguide are conventional IC fabrication steps. The conventional fabrication steps contribute to the reliability and yield of the process, while allowing optical waveguides to be incorporated in electrical IC-like structures as well as in purely optical IC-like structures. The waveguide is capable of being fabricated using self aligning techniques, which also contribute to the ease and reliability in fabrication.
A more complete appreciation of the present invention and its scope, and the manner in which it achieves the above noted improvements, can be obtained by reference to the following detailed description of presently preferred embodiments of the invention taken in connection with the accompanying drawings, which are briefly summarized below, and by reference to the appended claims.